# std::ranges::is_sorted_until

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< cpp‎ | algorithm‎ | ranges

Defined in header `<algorithm>` |  |
Call signature |  |
template< std::forward_iterator I, std::sentinel_for<I> S, class Proj = std::identity, std::indirect_strict_weak_order<std::projected<I, Proj>> Comp = ranges::less > constexpr I is_sorted_until( I first, S last, Comp comp = {}, Proj proj = {} ); | (1) | (since C++20)
template< std::forward_range R, class Proj = std::identity, std::indirect_strict_weak_order< std::projected<ranges::iterator_t<R>, Proj>> Comp = ranges::less > constexpr ranges::borrowed_iterator_t<R> is_sorted_until( R&& r, Comp comp = {}, Proj proj = {} ); | (2) | (since C++20)

Examines the range `[`first`,` last`)` and finds the largest range beginning at first in which the elements are sorted in non-descending order.

A sequence is sorted with respect to a comparator comp if for any iterator `it` pointing to the sequence and any non-negative integer `n` such that `it + n` is a valid iterator pointing to an element of the sequence, std::invoke(comp, std::invoke(proj, *(it + n)), std::invoke(proj, *it)) evaluates to false.

1) Elements are compared using the given binary comparison function comp.
2) Same as (1), but uses r as the source range, as if using ranges::begin(r) as first and ranges::end(r) as last.

The function-like entities described on this page are algorithm function objects (informally known as niebloids), that is:

- Explicit template argument lists cannot be specified when calling any of them.
- None of them are visible to argument-dependent lookup.
- When any of them are found by normal unqualified lookup as the name to the left of the function-call operator, argument-dependent lookup is inhibited.

### Parameters

first, last | - | iterator-sentinel defining the range to find its sorted upper bound
r | - | the range to find its sorted upper bound
comp | - | comparison function to apply to the projected elements
proj | - | projection to apply to the elements

### Return value

The upper bound of the largest range beginning at first in which the elements are sorted in non-descending order. That is, the last iterator `it` for which range `[`first`,` it`)` is sorted.

### Complexity

Linear in the distance between first and last.

### Possible implementation

    struct is_sorted_until_fn
    {
        template<std::forward_iterator I, std::sentinel_for<I> S, class Proj = std::identity,
                 std::indirect_strict_weak_order<std::projected<I, Proj>> Comp = ranges::less>
        constexpr I operator()(I first, S last, Comp comp = {}, Proj proj = {}) const
        {
            if (first == last)
                return first;

            for (auto next = first; ++next != last; first = next)
                if (std::invoke(comp, std::invoke(proj, *next), std::invoke(proj, *first)))
                    return next;

            return first;
        }

        template<ranges::forward_range R, class Proj = std::identity,
                 std::indirect_strict_weak_order<
                     std::projected<ranges::iterator_t<R>, Proj>> Comp = ranges::less>
        constexpr ranges::borrowed_iterator_t<R>
            operator()(R&& r, Comp comp = {}, Proj proj = {}) const
        {
            return (*this)(ranges::begin(r), ranges::end(r), std::ref(comp), std::ref(proj));
        }
    };

    inline constexpr is_sorted_until_fn is_sorted_until;

### Notes

`ranges::is_sorted_until` returns an iterator equal to last for empty ranges and ranges of length one.

### Example

Run this code

    #include <array>
    #include <algorithm>
    #include <iostream>
    #include <iterator>
    #include <random>

    int main()
    {
        std::random_device rd;
        std::mt19937 g {rd()};
        std::array nums {3, 1, 4, 1, 5, 9};

        constexpr int min_sorted_size = 4;
        int sorted_size = 0;
        do
        {
            std::ranges::shuffle(nums, g);
            const auto sorted_end = std::ranges::is_sorted_until(nums);
            sorted_size = std::ranges::distance(nums.begin(), sorted_end);

            std::ranges::copy(nums, std::ostream_iterator<int>(std::cout, " "));
            std::cout << " : " << sorted_size << " leading sorted element(s)\n";
        }
        while (sorted_size < min_sorted_size);
    }

Possible output:

    4 1 9 5 1 3  : 1 leading sorted element(s)
    4 5 9 3 1 1  : 3 leading sorted element(s)
    9 3 1 4 5 1  : 1 leading sorted element(s)
    1 3 5 4 1 9  : 3 leading sorted element(s)
    5 9 1 1 3 4  : 2 leading sorted element(s)
    4 9 1 5 1 3  : 2 leading sorted element(s)
    1 1 4 9 5 3  : 4 leading sorted element(s)

### See also

ranges::is_sorted (C++20) | checks whether a range is sorted into ascending order (algorithm function object)
is_sorted_until (C++11) | finds the largest sorted subrange (function template)

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